Fuel cell

ABSTRACT

A fuel cell comprises a cathode (electrode catalyst layer), an anode (electrode catalyst layer), and a gas diffusion layer provided at least on one of the cathode and anode between a collector and the electrode catalyst layer and containing carbon having an oil absorption larger than that of a catalyst-supporting carbon used in the electrode catalyst layers. As a result, the dropping of the cell voltage due to external factor is suppressed, even if the cell temperature is low (room temperature to 50° C.). The water-absorption pressure of the water passage in the gas diffusion layer is higher than those of the electrode catalyst layers. The produced water and moving water in the cathode is absorbed into the water passage in the gas diffusion layer exhausted to the collector side. When a cell temperature is low, the evaporation speed of water from the collectors is lowered. However, the produced water and moving water in the cathode are physically attracted on the basis of the water-absorbing pressure of the gas diffusion layer and discharged toward the collector side, so that the reduction of the cell voltage is prevented.

FIELD OF THE INVENTION

The present invention relates to a fuel cell, and more particularlyrelates to a polymer electrolyte fuel cell having an improved gasdiffusion layer for generating power by using a humidifying reactantgas.

Background Art

A fuel cell denotes a cell for directly taking out the chemical energyof a fuel (such as hydrogen) as electrical energy on the basis of anelectrochemical reaction. This is the basic principle of a fuel cell.Several types of fuel cell are used in accordance with a mode forrealizing the basic principle. The following types of cells aregenerally listed: an alkaline fuel cell, a polymer electrolyte fuelcell, a phosphoric-acid fuel cell, a molten-carbonate fuel cell, and asolid-oxide fuel cell.

A polymer electrolyte fuel cell (hereafter referred to as PEFC) isdescribed below, which is one of the above fuel cells. FIG. 4 is across-sectional view of a cell 1 of a PEFC. A gas diffusion layer 13 isinterposed between an anode-side collector 12 and an anode (electrodecatalyst layer) 14, and a gas diffusion layer 23 is interposed between acathode-side collector 22 and a cathode (electrode catalyst layer) 24.An electrolyte membrane 10 is a cation-exchange membrane. The anode-sidecollector 12 (cathode-side collector 22) is carbon paper containing 30%by mass of a tetrafluoroethylene-hexafluoropropylene copolymer (FEP), inwhich carbon paper is for example, impregnated with a 60%-by-masssolution of the FEP, then heat-treated at 360° C. for one hour, andformed into a predetermined dimension. The anode 14 (cathode 24) is madeof a mixture of catalyst-supporting carbon and an ion-conductingmaterial.

The gas diffusion layers 13 and 23 are a layer formed by applying awater-repelling treatment to carbon paper with fluorocarbon resin or thelike, a layer formed by mixing carbon and a hydrophobic material such asfluorocarbon resin to mold, or a layer formed by applying awater-repelling treatment to a mixture of carbon and a hydrophobicmaterial such as fluorocarbon resin on the surface of carbon papertreated for water-repellent with fluorocarbon resin or the like.

A PEFC supplies a reactant gas respectively to the above anode 14 andcathode 24, and generates power on the basis of oxidation reaction andreduction reaction of the following electrochemical reactions 1 and 2.H₂→2H⁺+2e ⁻  (Electrochemical reaction 1)2H⁺+1/20₂+2e ⁻→H²O  (Electrochemical reaction 2)

The PEFC is characterized in that a high output can be obtained thoughthe operating temperature is relatively low compared to those of othertypes of fuel cells.

In the case of the PEFC, when a fuel and an oxidizing gas are introducedinto the anode side and cathode side respectively, H+ (hereafterreferred to as proton) is generated in the anode 14 on the basis of theelectrochemical reaction 1 and moved in the electrolyte membrane 10 fromthe anode side to the cathode side together with water. In the cathode24, water is produced by protons and oxygen on the basis of theelectrochemical reaction 2. In the case of the PEFC, electrical energycan be obtained by taking out electrons generated together with protonsfrom the anode 14 and supplying them to the cathode 24. Moreover, sincethe electrolyte membrane 10 shows conductivity while it is wet, areactant gas is humidified and supplied to the cell 1 in general.

The following four functions are requested for the gas diffusion layers13 and 23 of the cell 1.

The first point is a gas diffusion property (permeability). This is afunction indispensable for a reactant gas to pass through the gasdiffusion layers 13 and 23, efficiently come into contact with theelectrode catalysts of the electrode catalyst layers 14 and 24, andcause an electrochemical reaction.

The second point is a water-repelling function. When the gas diffusionlayers 13 and 23 respectively have a high hygroscopicity, the gasdiffusion property is deteriorated because moisture in a reactant gashumidified and supplied and reaction-produced water are absorbed underPEFC power generation. Therefore, in order to realize a high-performancePEFC, water-repelling function of the gas diffusion layers 13 and 23 isalso indispensable.

The third point is an electron-conducting function. This is anindispensable function for delivering electrons generated on the basisof an electrochemical reaction to an external circuit and taking inelectrons from an external circuit.

The fourth point is a water-holding property. When the water-holdingproperty is deteriorated, the electrode catalyst layers 14 and 24 or theelectrolyte membrane 10 are or is dried and the ion conductivity isdeteriorated. Therefore, the water-holding property is alsoindispensable in order to realize a high-performance PEFC.

So as to provide with the above basic four functions, a layer formed byapplying a water-repelling treatment to carbon paper with fluorocarbonresin or the like is used for the conventional gas diffusion layers 13and 23 respectively.

However, a PEFC tends to greatly change in the direction in which a cellvoltage is lowered due to fluctuations in external factors such as ahumidified degree of a reactant gas, a quantity of a reactant gas to besupplied, and a cell temperature under power generation. In the case ofthe cell 1 using the conventional gas diffusion layers 13 and 23 formedby applying a water-repelling treatment to carbon paper withfluorocarbon resin, a problem is caused that a cell voltage tends toremarkably lower under a low temperature (room temperature to 50° C.).

Therefore, it is an object of the present invention to provide a fuelcell having a cell provided with a gas diffusion layer capable ofpreventing a cell voltage from being lowered due to external factors byimproving a conventional gas diffusion layer.

DISCLOSURE OF THE INVENTION

As a result of performing earnest study in order to solve theconventional problems, the present inventor et al. find that by using agas diffusion layer containing carbon having a specific oil absorptionvolume, preferably by using a gas diffusion layer having a newstructure, it is possible to prevent a cell voltage from being lowereddue to external factors and prevent a cell voltage from being loweredalso when a cell temperature is low (room temperature to 50° C.) andrealize the present invention.

That is, claim 1 of the present invention discloses a fuel cellconstituted by arranging an anode-side collector, an anode (electrodecatalyst layer), an electrolyte membrane, a cathode (electrode catalystlayer), and a cathode-side collector, characterized in that a gasdiffusion layer containing carbon having an oil absorption volume largerthan that of the catalyst-supporting carbon used for the electrodecatalyst layers is interposed between the collectors and the electrodecatalyst layers on at least either the cathode (electrode catalystlayer) or the anode (electrode catalyst layer).

claim 2 of the present invention is characterized in that concaves andconvexes are formed on at least one side face of the gas diffusion layerin the fuel cell of claim 1.

claim 3 of the present invention is characterized in that the gasdiffusion layer comprises a hydrophobic material and the carbon in thefuel cell of claim 1 or 2.

claim 4 of the present invention is characterized in that the content ofthe hydrophobic material ranges from 0.5% to 50% by mass in the fuelcell of claim 3.

claim 5 of the present invention is characterized in that thehydrophobic material is selected from a group composed ofpolytetrafluoroethylene (PTFE),tetrafluoroethylene-perfluoroalkylvinly-ether copolymer (PFA),tetrafluoroethylene-hexafluoropropylene copolymer (FEP),polycholotrifuoroethylene (PCTFE), polyvinylidene fluoride (PVDF),polyvinyl fluoride (PVF), and tetrafluoroethylene-ethylene copolymer(ETFE) in the fuel cell of claim 3 or 4.

claim 6 of the present invention is characterized in that the intervalsbetween concaves and convexes range between 55 and 200 μm in the fuelcell of claim 2.

A PEFC having a cell provided with an improved gas diffusion layer towhich the present invention is applied is specifically described belowby referring to the accompanying drawings.

FIG. 1 is a cross-sectional view of a cell 1A of a PEFC of the presentinvention. A gas diffusion layer 13 is interposed between an anode-sidecollector 12 and an anode (electrode catalyst layer) 14 and, a gasdiffusion layer 23 is interposed between a cathode-side collector 22 anda cathode (electrode catalyst layer) 24. The gas diffusion layers 13 and23 comprise layers containing carbon having an oil absorption volumelarger than that of the catalyst-supporting carbon of the electrodecatalyst layers 14 and 24. As described above, in a fuel cell using theconventional gas diffusion layers 13 and 23 shown in FIG. 4 respectivelyobtained by applying a mixture of carbon and a hydrophobic material suchas fluorocarbon resin to the surface of carbon paper treated forwater-repellent with fluorocarbon resin or the like, as the examinedtest of causes for a cell voltage to lower, the present inventor et alfound that one of the causes was stay of reaction-produced water,condensed water, moving water, or inversely-diffused water in theelectrodes (electrode catalyst layers) 14 and 24 due to a factor such asa cell temperature or on the contrary, or drying of the electrodes(electrode catalyst layers) 14 and 24 and the electrolyte membrane 10.

That is, it was estimated that, when a cell temperature was too low orwater was excessively supplied to the cell 1, supply of a reactant gasto the electrodes (electrode catalyst layers) 14 and 24 was preventeddue to reaction-produced water, moving water, or inversely-diffusedwater stayed in the electrodes (electrode catalyst layers) 14 and 24 andthereby a cell voltage was lowered. On the contrary, when a celltemperature was too high or the quantity of supplied water was toosmall, the water volume held in the cell 1 becomes insufficient, theelectrodes (electrode catalyst layers) 14, 24 and the electrolytemembrane 10 were dried and the cell voltage was lowered.

As a result of using the gas diffusion layers 13 and 23 containingcarbon having an oil absorption volume larger than that of thecatalyst-supporting carbon of the electrodes (electrode catalyst layers)14 and 24 on the basis of the above knowledge, it was possible tosuppress lowering of a cell voltage due to external factors. This isbecause stay of water in the electrodes (electrode catalyst layers) 14and 24 or drying of the electrodes (electrode catalyst layers) 14 and 24and the electrolyte membrane 10 due to fluctuations of external factorsis suppressed by the gas diffusion layers 13 and 23 used for the presentinvention.

Reaction-produced water tends to stay in the electrode (electrodecatalyst layer) 24 at the cathode side. Therefore, by arranging the gasdiffusion layer 23 used for the present invention to at least thecathode (electrode catalyst layer) 24, the effect thereof remarkablyappears.

It is preferable that the gas diffusion layers 13 and 23 used for thepresent invention comprise carbon having an oil absorption volume largerthan that of the catalyst-supporting carbon of the electrodes (electrodecatalyst layers) 14 and 24, and a hydrophobic material. By combining theformer carbon with the hydrophobic material, it is possible to arrange acapillary water passage, a water-holding portion, a water-repellingcapillary gas passage, and an electron-conducting passage in the gasdiffusion layers 13 and 24.

It is preferable that the content of the hydrophobic material is 0.5% to50% by mass and more preferable that the content of the hydrophobicmaterial is 1% to 40% by mass. When the content of the hydrophobicmaterial is less than 0.5% by mass, a water-repelling capillary gas pathmay not be completely formed. However, when the content of thehydrophobic material exceeds 50% by mass, a capillary water passage andan electron-conducting passage or a water-holding portion may not becompletely formed.

As a specific preferable hydrophobic material capable of forming awater-repelling capillary gas path, fluorocarbon resin selected from thefollowing materials can be used: polytetrafluoroethylene (PTFE),tetrafluoroethylene-perfluoroalkylvinly-ether copolymer (PFA),tetrafluoroethylene-hexafluoropropylene copolymer (FEP),polycholotrifuoroethylene (PCTFE), polyvinylidene fluoride (PVDF),polyvinyl fluoride (PVF), and tetrafluoroethylene-ethylene copolymer(ETFE).

As a specific example of the preferable carbon for forming a capillarywater passage and an electron-conducting passage or a water-holdingportion, the following can be used: carbon powder such as furnace black(Vulcan XC-72 [made by Cabot Inc.] as typical product) or acetyleneblack (DENKA black [made by DENKI KAGAKU KOGYO K.K.] as typicalproduct).

The anode-side collector 12 (cathode-side collector 22) uses carbonpaper having an FEP content of 30% by mass obtained by impregnating thecarbon paper with 60% by mass of tetrafluoroethylene-hexafluoropropylenecopolymer (FEP) and then heat-treating the paper at 360° C. for one hourand forming the paper into predetermined dimensions.

The anode 14 (cathode 24) is made of a mixture of catalyst-supportingcarbon and an ion-conducting material. The electrolyte membrane 10 is acation-exchange membrane.

Even when water is excessively supplied to the cathode side, thediffusion passage of a reactant gas is secured by a capillary gaspassage defined by a hydrophobic material in the gas diffusion layer 23composed of carbon having an oil absorption volume larger than that ofthe catalyst-supporting carbon of the electrode (electrode catalystlayer) 24 and the hydrophobic material. However, since the capillarywater passage defined by the carbon is formed by using the carbon havingan oil absorption volume larger than that of the catalyst-supportingcarbon, the water-absorbing pressure of the water passage in the gasdiffusion layer 23 is higher than that of the catalyst-supporting carbonof the cathode (electrode catalyst layer) 24 and the produced water andmoving water in the cathode (electrode catalyst layer) are attractedinto the water passage in the gas diffusion layer 23 and dischargedtoward the cathode-side collector 22.

When a cell temperature is low, the evaporation speed of water from thecollectors 12 and 22 is lowered. However, the produced water and movingwater in the cathode (electrode catalyst layer) 24 are physicallyattracted on the basis of the water-absorbing pressure of the gasdiffusion layer 23 and discharged toward the cathode-side collector 22.

As shown in FIG. 1, when forming concaves and convexes 25 on theinterface between the cathode (electrode catalyst layer) 24 and the gasdiffusion layer 23 and the interface between the anode (electrodecatalyst layer) 14 and the gas diffusion layer 13 and increasing thecontact area between them in order to make the water-absorbing pressuredifference more effectively function, discharge performances of producedwater and moving water in the cathode (electrode catalyst layer) 24 arefurther improved.

Moreover, as shown in FIG. 1, by forming concaves and convexes 26 at thecollectors 12 and 22 of the gas diffusion layers 13 and 23 in order toincrease the evaporation speed of water from the gas diffusion layers 13and 23 even when a cell temperature is low as shown in FIG. 1, it ispossible to increase an evaporation area and improve the dischargeperformance to the outside of the cell 1A.

It is preferable that the interval between the concaves or convexes 25and the interval between the concaves or convexes 26 respectively rangesfrom 55 to 200 μm and more preferable that the intervals respectivelyrange from 90 to 150 μm. The interval between concaves or convexes 25and the interval between the concaves or convexes 26 respectively denotethe interval between a convex and the next convex or the intervalbetween a concave and the next concave. When forming the concaves andconvexes 25 and concaves and convexes 26 by pressing a screen againstthem as the case of an embodiment to be described later, the intervalbetween the concaves and convexes 25 and the interval between theconcaves and convexes 26 respectively correspond to the interval(opening) between lines of a screen (net) used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a cell of the polymer electrolytefuel cell of an embodiment;

FIG. 2 is an illustration of graphs showing relations of cell voltagesto current densities of the polymer electrolyte fuel cell of anembodiment of the present invention and the polymer electrolyte fuelcell of a comparative example;

FIG. 3 is an illustration of graphs showing relations of cell voltagesto current densities of the polymer electrolyte fuel cell of anembodiment of the present invention and the polymer electrolyte fuelcell of a comparative example under another test condition; and

FIG. 4 is a cross-sectional view of a cell of a conventional polymerelectrolyte fuel cell.

In FIGS. 1 and 4, symbols 1 and lA denote cells, 10 denotes anelectrolyte membrane, 12 denotes an anode-side collector, 13 and 23denote gas diffusion layers, 14 denotes an anode (electrode catalystlayer), 22 denotes a cathode-side collector, 24 denotes a cathode(electrode catalyst layer), and 25 and 26 denote concaves and convexes.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is more minutely described below in accordancewith embodiments and comparative examples. However, the presentinvention is not restricted to the embodiments.

(Embodiment 1)

<Fabrication of Collectors 12 and 22>

The collectors 12 and 22 are fabricated by impregnating the collectors12 and 22 with a 60%-by-mass solution oftetrafluoroethylene-hexafluoropropylene copolymer (FEP), thenheat-treating the collectors 12 and 22 at 360° C. for one hour, andforming them into predetermined dimensions.

<Formation of Gas Diffusion Layers 13 and 23>

The gas diffusion layers 13 and 23 are formed by forming a compositionprepared by mixing BLACK PEARLS 2000 having an oil absorption volume of330 cc/100 g, a 60%-by-mass PTFE solution, and kerosene so that the PTFEcontent becomes 30% by mass on the collectors 12 and 22 on the basis ofthe publicly-known screen printing method.

When performing screen-printing, the concaves and convexes 26 are formedon the gas diffusion layers 13 and 23 at the collectors 12 and 22 bymaking the composition enter the collectors 12 and 22. Therefore,printing is performed while performing attracting from the back of ascreen printing face. Moreover, after performing screen printing, theconcaves and convexes 25 are formed at the electrode catalyst layers 14and 24 by pressing a 100-mesh screen from upper portions of the printingfaces of the gas diffusion layers 13 and 23. Then, the collectors 12 and22 provided with the gas diffusion layers 13 and 23 are fabricated byheat-treating the collectors 12 and 22 at 360° C. for one hour andforming them into predetermined dimensions.

<Formation of Electrode Catalyst Layers 14 and 24>

The anode (electrode catalyst layer) 14 and cathode (electrode catalystlayer) 24 are formed on the collectors 12 and 22 provided with the gasdiffusion layers 13 and 23 by forming platinum-supporting carbon (Pt/C)and Nafion (trade name) into a composition having a Nafion (trade name)quantity of 10% by mass and using the composition and the publicly-knownscreen printing method.

The carbon made of the platinum-supporting carbon (Pt/C) uses VulcanXC-72 (oil absorption volume: 174 cc/100 g) having an oil absorptionvolume smaller than that of BLACK PERRLS 2000 having an oil absorptionvolume of 330 cc/100 g used for the gas diffusion layers 13 and 23.

While bringing the electrodes (electrode catalyst layers) 14 and 24 intocontact with the membrane 10, the cell 1A is formed by contact-bondingthem on the basis of the hot pressing method (temperature of 150° C.,pressure of 6.9 MPa, and treatment time of 90 sec).

<Experiment 1>

FIG. 2 shows a result of performing a test for obtaining current-voltagecharacteristics under the following cell-testing conditions by using thecell 1A thus formed.

(Cell-Testing Conditions)

-   -   Electrode area: 25 cm²    -   Fuel: H₂    -   Oxidizing-gas: air    -   Cell temperature: 50° C.    -   Fuel-humidifying temperature: 50° C.    -   Oxidizing-gas-humidifying temperature: 50° C.

<Experiment 2>

FIG. 3 shows a result of performing a test for obtaining current-voltagecharacteristics under the following cell-testing conditions by using thecell 1A.

(Cell-Testing Conditions)

-   -   Electrode area: 25 cm²    -   Fuel: H₂    -   Oxidizing-gas: air    -   Cell temperature: 80° C.    -   Fuel-humidifying temperature: 80° C.    -   Oxidizing-gas-humidifying temperature: 85° C.

COMPARATIVE EXAMPLE 1

A cell 1B for comparison is formed under the same conditions as the caseof the embodiment 1 except that the gas diffusion layers 13 and 23respectively use BLACK PEARLS 800 (oil absorption volume: 68cc/100 g)having an oil absorption volume smaller than that of the Vulcan XC-72(oil absorption volume: 174cc/100 g) used for the platinum-supportingcarbon (Pt/C).

FIGS. 2 and 3 show results of obtaining current-voltage characteristicsby using the cell 1B and thereby performing the experiments 1 and 2similarly to the case of the embodiment 1.

From FIG. 2, it is found that the cell 1A shows a cell voltage morepreferable than that of the cell 1B of the comparative example 1 evenunder an over-humidified state in which a cell temperature is a lowtemperature of 50° C. and a fuel-humidifying temperature and ahumidified quantity of an oxidizing gas are 50° C. Particularly, in thecase of the cell 1A of the embodiment 1, the lowering rate of a cellvoltage is small in a region having a large current density and thereby,the effect is remarkable.

From FIG. 3, it is found that the cell 1A of the embodiment 1 shows acell voltage more preferable than that of the cell 1B of the comparativeexample 1 even under an over-humidified state in which anoxidizing-gas-humidifying temperature is 85° C. to a cell temperature of80° C. Particularly, the effect of the cell 1A of the embodiment 1 isremarkable because the lowering rate of a cell voltage in a regionhaving a large current density is small.

This embodiment uses Vulcan XC-72 (oil absorption volume: 174cc/100 g)for the electrodes (electrode catalyst layers) 14 and 24. For example,however, when using BLACK PEARLS 1100 (oil absorption volume: 500cc/100g) for electrodes (electrode catalyst layers 14 and 24), it is possibleto use BLACK PEARLS 800 (oil absorption volume: 68cc/100 g) for the gasdiffusion layers 13 and 23. Thus, it is possible to properly select acombination of types of carbon used for the gas diffusion layers 13 and23 and the electrodes (electrode catalyst layers) 14 and 24.

<Material of Anode Catalyst>

Moreover, for this embodiment, a case is described in which simplexplatinum (Pt) is used as an anode catalyst. However, it is also allowedto use ruthenium (Ru) and moreover, it is allowed to use one of generalcatalyst materials such as gold (Au), silver (Ag), palladium (Pd), andrhodium (Rh) or an alloy of these materials. Furthermore, it is allowedto use an alloy obtained by adding one of the catalyst materials such asiron (Fe), nickel (Ni), chromium (Cr), molybdenum (Mo), iridium (Ir)gallium (Ga), titanium (Ti), vanadium (V), aluminum (Al), and tin (Sn)to the catalyst material.

<Material of Cathode Catalyst>

Furthermore, for this embodiment, a case is described in which simplexplatinum (Pt) is used for a cathode catalyst. However, it is alsoallowed to use an alloy obtained by adding one of the catalyst materialssuch as nickel (Ni), iron (Fe), copper (Cu), chromium (Cr), vanadium(V), gold (Au), silver (Ag), palladium (Pd), rhodium (Rh), iridium (Ir),gallium (Ga), titanium (Ti), aluminum (Al), and tin (Sn).Moreover, this embodiment uses a PEFC as a fuel cell of the presentinvention. However, a fuel cell of the present invention is notrestricted to the PEFC but it includes an alkaline fuel cell and aphosphoric-acid fuel cell.

(Embodiment 2)

A cell 1C is formed similarly to the case of the embodiment 1 exceptthat a 100-mesh screen (net) is not used or concaves and convexes 25 arenot formed on the gas diffusion layers 13 and 23.

<Experiment 3>

Table 1 shows results of performing tests for obtaining cell voltagesunder the following cell-testing conditions by using the cell 1C thusformed. Moreover, Table 1 shows inch meshes of screens (nets) used, linediameters (μm), thicknesses (μm) and openings (μm).

(Cell-Testing Conditions)

-   -   Electrode area: 25 cm²    -   Fuel: H₂    -   Oxidizing-gas: air    -   Cell temperature: 50° C.    -   Fuel-humidifying temperature: 50° C.    -   Oxidizing-gas-humidifying temperature: 50° C.    -   Current density: 500 mA/cm²

(Comparative Example 2)

Table 1 shows results of performing tests for obtaining cell voltages ofthe cell 1B formed for the comparative example 1 under the cell-testingconditions of the above experiment 3.

(Embodiments 3 to 10)

Table 1 shows results of performing tests for obtaining cell voltagesunder the cell-testing conditions of the above experiment 3 by formingcells similarly to the case of the embodiment 1 except that screens(nets) of 50 to 400 meshes are used.

TABLE 1 Line Thick- Cell Inch diameter ness Opening voltage mesh (μm)(μm) (μm) (mV) Comparative 100 101 225 153 380 example 2 Embodiment 2 —— — — 570 Embodiment 3 50 193 415 315 570 Embodiment 4 60 115 275 271570 Embodiment 5 80 121 237 196 580 Embodiment 6 100 101 225 153 590Embodiment 7 200 40 84 87 590 Embodiment 8 250 35 73 87 580 Embodiment 9300 30 70 55 580 Embodiment 10 400 23 52 41 570

From Table 1, it is found that when using Vulcan XC-72 (oil absorptionvolume: 174 cc/100 g) having an oil absorption volume smaller than thatof the BLACK PERRLS 2000 having an oil absorption volume of 300 cc/100 gused for a gas diffusion layer as the carbon used forplatinum-supporting carbon (Pt/C), a cell voltage of even the embodiment2 not provided with concaves and convexes by a screen (net) is improvedcompared to the case of the comparative example 2. In the case of theembodiments 3 to 10, a cell voltage is improved compared to the case ofthe comparative example 2. Moreover, it is found that a cell voltage isfurther improved in the case of the embodiment 5 (80 mesh) to embodiment9 (300 mesh) and a cell voltage is still further improved in the case ofthe embodiment 6 (100 mesh) to embodiment 7 (200 mesh).

In the case of the fuel cell of claim 1 of the present invention, a gasdiffusion layer containing the carbon having an oil absorption volumelarger than that of the catalyst-supporting carbon used for an electrodecatalyst layer between a collector and an electrode catalyst layer isformed on at least either a cathode (electrode catalyst layer) or ananode (electrode catalyst layer). Therefore, it is possible to prevent acell voltage from being lowered due to an external factor and prevent acell voltage from being lowered even when a cell temperature is low(room temperature to 50° C.).

Moreover, in the case of the fuel cell of claim 1 of the presentinvention, the water-absorbing pressure of the water passage of a gasdiffusion layer is higher than that of an electrode catalyst layer andfor example, the produced water and moving water in a cathode areattracted by a gas diffusion-layer water passage and discharged toward acollector. When a cell temperature is low, the evaporation speed ofwater from the collector side is lowered. For example, however, producedwater and moving water in a cathode are physically attracted by thewater-absorbing pressure of a gas diffusion layer and discharged towarda collector. Therefore, an advantage is obtained that it is possible toprevent a cell voltage from lowering.

In the case of the fuel cell of claim 2 of the present invention, sinceconcaves and convexes are formed at least one side face of the gasdiffusion layer, advantages are obtained that it is possible to increasethe contact area between the gas diffusion layer and an electrodecatalyst layer or between the gas diffusion layer and a collector, thedischarge performances of produced water and moving water in anelectrode catalyst layer are further improved, and it is possible toincrease the evaporation speed of water from the gas diffusion layereven if a cell temperature is low and improve the discharge performanceof water to the outside of a cell.

In the case of the fuel cell of claim 3 of the present invention, thegas diffusion layer comprises a hydrophobic material and the carbon.Therefore, even if water is excessively supplied to the cathode side,the diffusion passage of a reactant gas is secured by a capillary gaspassage defined by a hydrophobic material. Since a capillary waterpassage is defined by carbon having an oil absorption volume larger thanthat of catalyst-supporting carbon, the water-absorbing pressure of agas diffusion-layer water passage is higher than that of a cathode, andproduced water and moving water in the cathode are attracted by the gasdiffusion-layer water passage and discharged toward a collector.Moreover, when a cell temperature is low, the evaporation speed of waterfrom the collector side is lowered. However, the produced water andmoving water in the cathode are physically attracted by thewater-absorbing pressure of the gas diffusion layer and discharge to thecollector side. Furthermore, even if a cell temperature is low, anadvantage is obtained that it is possible to prevent a cell voltage fromlowering.

In the case of the fuel cell of claim 4 of the present invention, sincethe content of the hydrophobic material is set in a proper range from0.5% to 50% by mass. Therefore, a repellent capillary gas passage iscompletely formed and moreover, a capillary water passage, anelectron-conducting passage, a water-holding portion are completelyformed. Therefore, for example, even if water is excessively supplied tothe cathode side, a reactant gas diffusion passage is more-properlysecured by a capillary gas passage defined by a hydrophobic material andthe water absorbing pressure of a gas diffusion-layer water passagebecomes higher than that of a cathode and the produced water and movingwater in the cathode are attracted by the gas diffusion-layer waterpassage and more properly discharged to the collector side because acapillary water passage is defined by the carbon having an oilabsorption volume larger than that of catalyst-supporting carbon. When acell temperature is low, the evaporation speed of water from thecollector side is lowered. However, because the produced water andmoving water in a cathode are physically attracted due to thewater-absorbing pressure of a gas diffusion layer and more properlydischarged by the collector side. Therefore, an advantage is obtainedthat it is possible to prevent a cell voltage from lowering even if thecell temperature is low.

The fuel cell of claim 5 of the present invention is easily availableand economical because the hydrophobic material is selected from a groupcomposed of polytetrafluoroethylene (PTFE),tetrafluoroethylene-perfluoroalkylvinly-ether copolymer (PFA),tetrafluoroethylene-hexafluoropropylene copolymer (FEP),polycholotrifuoroethylene (PCTFE), polyvinylidene fluoride (PVDF),polyvinyl fluoride (PVF), and tetrafluoroethylene-ethylene copolymer(ETFE).

In the case of the fuel cell of claim 6 of the present invention,intervals between the concaves and convexes range from 55 to 200 μm.Therefore, it is possible to efficiently increase the contact areabetween a gas diffusion layer and an electrode catalyst layer or betweenthe gas diffusion layer and a collector, discharge performances of theproduced water and moving water in the electrode catalyst layer arefurther improved. Moreover, when a cell temperature is low, it ispossible to further increase the evaporation speed of water from the gasdiffusion layer and further improve the discharge performance of waterto the outside of a cell.

INDUSTRIAL APPLICABILITY

A conventional fuel cell tends to greatly change in the direction for acell voltage to lower due to fluctuations of external factors such as aquantity of humidified reactant gas under power generation, a quantityof reactant reactive gas to be supplied, and a cell temperature. In thecase of a cell using a conventional gas diffusion layer formed byapplying a water-repelling treatment to carbon paper with fluorocarbonresin, a cell voltage tends to lower at a low temperature (roomtemperature to 50° C.). In the case of a fuel cell of the presentinvention, however, it is possible to prevent a cell voltage from beinglowered due to external factors because the fuel cell has a cellobtained by improving a conventional gas diffusion layer so as to beable to prevent the cell voltage from being lowered due to externalfactors. Even if a cell temperature is low (room temperature to 50° C.),an advantage capable of preventing a cell voltage from being lowered isobtained and therefore, a fuel cell of the prevent invention has avery-large industrial utilization value.

1. A fuel cell comprising: an anode-side collector, an anode comprisingan electrode catalyst layer containing catalyst-supporting carbon, anelectrolyte membrane, a cathode comprising an electrode catalyst layercontaining catalyst-supporting carbon, a cathode-side collector arrangedin the order stated, further including a gas diffusion layer containingcarbon having an oil absorption volume per unit weight of carbon whichis larger than an oil absorption volume per unit weight of the carbon ofthe catalyst-supporting carbon of at least one of the anode and thecathode, the gas diffusion layer being interposed between a collectorand at least one of the cathode and the anode and having concaves andconvexes formed on at least one face thereof, wherein intervals betweensaid concaves and convexes are constant and are in the range of 55 to200 μm.
 2. The fuel cell according to claim 1, wherein said gasdiffusion layer comprises a hydrophobic material and said carbon.
 3. Thefuel cell according to claim 2, wherein a content of said hydrophobicmaterial ranges from 0.5% to 50% by mass.
 4. The fuel cell according toclaim 2 or 3, wherein said hydrophobic material is selected from a groupcomposed of polytetrafluoroethylene (PTFE),tetrafluoroethylene-perfluoroalkylvinyl-ether copolymer (PFA),tetrafluoroethylene-hexafluoropropylene copolymer (FEP),polychlorotrifluoroethylene (PCTFE), polyvinylidene fluoride (PVDF),polyvinyl fluoride (PVF), and tetrafluoroethylene-ethylene copolymer(ETFE).